Some manufacturers can provide (special order) an adaptor that plugs into the proprietary "CD/Changer" input to allow use of third-party (Empeg!) line-level inputs.

I'd visit a car audio supplier and ask about that before adding extra amps or replacing head units.

In my Subaru, I just went ahead and replaced the factory head unit (stereo) with a JVC model that HAS line-level AUX inputs on the back. Pretty cheap (shop around), and it works better than the factory stereo did, and also has a built-in CD player .. handy.

-ml